It is known that permanent waves are created through two steps, i.e., reductive splitting of cystine (disulfide) bonds of hair by the action of a reducing agent, and subsequent neutralization or fixing with an oxidizing agent, whereby the cystine bonds are restored.
The permanent processing of hair has generally used the so-called keratin reducing substances such as thioglycolic acid, cysteine, acetylcysteine and salts thereof. These keratin reducing substances exhibit a practical performance for permanent hair processing under alkaline conditions, and therefore most permanent solutions are rendered alkaline with a pH of approximately 9.5. However, the alkaline permanent solutions are known to damage the hair and scalp. To solve such problems, keratin reducing substances usable at a neutral to weakly acidic pH range (pH: 3-7.5 at 25° C.) have been developed.
For example, monoglycerol esters of thioglycolic acid have been studied as the keratin reducing substances usable at the above pH range (for example, Patent Document 1). To solve skin problems caused by thioglycolic acid esters, mercaptoglycolic acid amide derivatives and mercaptolactic acid amide derivatives are studied (for example, Patent Documents 2 and 3). Further, cysteamines, which are believed to produce effects under weakly acidic conditions, are also studied (for example, Patent Document 4).    [Patent Document 1] JP-A-H08-291031    [Patent Document 2] JP-A-2000-507272    [Patent Document 3] JP-A-2003-528901    [Patent Document 4] JP-A-H03-271214